Sunday, May 4, 2014

Lightning in Exotic Superheated Atmospheres

In the Solar System, lightning activity is present on all
planets that have clouds in their atmospheres. With the sheer number and
diversity of exoplanets discovered in recent years, it is worth considering if
lightning activity is also present on these worlds. A study by Christiane
Helling et al. (2012) examined the possibility of lightning being present in
the exotic superheated atmospheres of hot-Jupiters and brown dwarfs.
Hot-Jupiters are giant gaseous planets that orbit very close to their host
stars, while brown dwarfs are objects that bridge the gap between the most
massive planets and the least massive stars.

Figure 1: Artist’s impression of a brown dwarf.

Both hot-Jupiters and brown dwarfs (provided the brown dwarf
has not cooled sufficiently) have atmospheric temperatures that are much higher
than on Earth or on Jupiter. Temperatures in these atmospheres can reach thousands
of degrees Kelvin. Such temperatures are too hot for water to exist outside the
vapour phase, thereby ruling out the moist thundercloud charging process that is
responsible for generating lightning here on Earth, and on the gas giants
Jupiter and Saturn. Nevertheless, the superheated atmospheres of hot-Jupiters
and brown dwarfs are dominated by mineral clouds, consisting of a number of
mineral species in the form of liquid mineral droplets and/or mineral dust
particles. For example, at sufficiently high temperatures, droplets of liquid
iron (Fe) or droplets of liquid titanium oxide (TiO2) can fall as rain.

Figure 2: Dust cloud material composition in volume
fractions in the atmosphere of a hot-Jupiter. The compositional changes with atmospheric
height are indicated by the local temperature. Source: Christiane Helling et
al. (2012).

In order for lightning to be present, the mineral clouds
need to undergo some form of ionisation. A number of mechanisms can lead to
sufficient ionisation of the mineral clouds. These mechanisms include energetic
radiation from the host star, triboelectric charging and cosmic rays. Of
particular interest is triboelectric charging, involving dust-dust collisions.
Although dust-dust collisions from gravitational settling (i.e. mineral dust
particles raining out) are not energetic enough to induce triboelectric
charging, atmospheric turbulence can enhance the dust-dust collisions such that
triboelectric charging becomes possible.

Nevertheless, gravitational setting enables large-scale
charge separation in the mineral clouds and establishes the condition necessary
for lightning to occur. The large-scale charge separation develops as particles
of different sizes fall at different rates. Smaller, less negatively charged
particles fall slower and remain longer in the upper cloud layers. Conditions
for lightning to occur are created when large-scale charge separation eventually
causes the electric potential between two regions of the mineral cloud to
exceed the breakdown field.

The study by Christiane Helling et al. (2012) concludes that
mineral clouds in the atmospheres of hot-Jupiters and brown dwarfs should
generate lightning since the breakdown fields are lower than on Earth or on
Jupiter. Hot-Jupiters and brown dwarfs do not have solid surfaces like on
terrestrial planets such as the Earth. As a result, lightning activity occurs
in the form of intra-cloud discharges.